The destructive effect of caries (i.g., acid-dissolution or demineralization) upon dental hard tissues is related to the physico-chemical properties of the mineral phase, which is an apatite. The absolute structure and composition of human enamel apatite is still in dispute partly due to the presence of some minor constituents (such as carbonate, fluoride, chloride, pyrophosphate, magnesium, etc.) and trace elements (e.g., zinc, lead, copper, strontium, etc) and their nebulous association with the apatite structure. Some of these constituents have been vaguely associated with caries-resistance or caries-susceptibility of teeth. The caries-resistant properties of the dental apatites are also thought to be related to the manner and conditions under which these apatites form. In this regard, these biological apatites have been reported to have a precursor-related calcium phosphate such as brushite, octacalcium phosphate, whitlockite and even amorphous calcium phosphate. The over-all objectives of this research are: to study the crytal:gr owth conditions of apatites and related calcium phosphates; to evaluate the effects of minor constituents on the physical (i.g., size, shape, unit-cell dimensions, strain) and chemical (i.e., bonding solubility) properties of apaties (using crystallographic and spectroscopic methods); to better define and characterize the structure of enamel in the light of information obtained from synthetic apatites; to evaluate the efficacy of the various anti-caries agents using biological and synthetic apatites.